This will also affect iOS 9 and older.
A comment on Howard’s story makes it sound easy to manually add the needed newer root certificate to OS X or to iOS:
You double-click the certificate, accept the prompts, type in your password, done. It’s not any more difficult even on any ancient iOS version.
That should work until that certificate expires, September 30, 2024.
For just web browsing on the Mac, one could also use Firefox 78.14.0esr, it uses its own list of root certificates which is recent enough and that specific version is the last to run on OS X 10.9 - 10.11.
Separate from certificate issues, Safari 9 and other old browsers will likely have more problems in the future. As support for Internet Explorer 11 is finally being dropped from web sites, the next oldest supported browser will likely be iOS Safari 10.3.